The present invention relates to a process for preparing rare earth-iron-boron alloy powders by a reduction/diffusion method. The present invention is also directed to the powders thus produced which are useful in permanent magnets and related technology. The present invention is also directed to articles which use the alloy powders produced by the instant process.
2. Background of the Prior Art
Alloys which contain rare earths, i.e., those elements with atomic numbers 57 to 71 as their principal components, are used in a variety of areas, including permanent magnets, magnetostrictive materials, opto-magnetic recording materials, hydrogen occlusion materials, and magnetic sensors.
Magnets utilizing these alloys exhibit excellent properties. One such alloy which has particular utility in permanent magnets, is formed from a rare earth, iron and boron. This alloy is generally described by the formula R--Fe--B, wherein R signifies one or more rare earth elements, Fe signifies iron, and B, boron. Two processes currently exist for the preparation of R--Fe--B alloy powders. The first process is a powder preparation method; the second process is a reduction/diffusion method. In the first method, which is a powder metallurgy process, ingots of rare earth metals and other alloying elements are melted using a high frequency melting furnace to form the R--Fe--B alloy which is subsequently crushed into powders. However, it is difficult to make the R--Fe--B alloy powders in this manner because the rare earth metals are easily oxidized during the crushing operation, which result adversely affects the quality of the final product.
To eliminate this drawback, a reduction/diffusion method has been developed. The starting materials for this method consist of rare earth metal oxides, iron powders, and ferroboron powders, all of which are admixed with calcium granules which act as a reducing agent. Cobalt powders and aluminum oxide may also be present in the starting materials. The mixture obtained is dry pressed and heated in either an inert gas atmosphere or under vacuum in order to reduce the rare earth metal oxide by contact with the resultant melt and/or by contact with the vapor coming from the calcium granules. The rare earth metal which is formed by this reduction then diffuses into the particles of ferroboron, iron (and cobalt and aluminum oxide, if these are present). While this method permits the formation of an R--Fe--B alloy powder having a uniform composition, it suffers the drawback of providing an impure product: the reaction product, obtained in the form of a sintered mass, is a mixture of calcium oxide, CaO, which is formed as a by product of reaction, unreacted excess calcium, and the desired R--Fe--B alloy powder.
When the sintered mass is crushed and placed into water, the CaO and the unreacted excess metallic calcium react with the water to form calcium hydroxide, Ca(OH).sub.2. The desired R--Fe--B alloy powder can then be separated from the Ca(OH).sub.2 because the Ca(OH).sub.2 remains suspended in the water, while the R--Fe--B alloy powder becomes a slurry which settles upon standing. The water containing the Ca(OH).sub.2 suspension is physically removed from the settled R--Fe--B alloy powder slurry by decantation, for example. Residual Ca(OH).sub.2 is removed by washing the R--Fe--B alloy powder slurry with an acid. Upon drying, the R--Fe--B alloy powder is obtained. Since rare earths in oxide form cost less than ingots of rare earth metals, as used in the powder metallurgy method, it is the reduction/diffusion method which is the subject of intense interest to those in the magnetic material industry.
Accordingly, the reduction/diffusion method has undergone extensive development since the use of rare earth-iron-boron ally materials, such as neodymium-iron-boron (Nd--Fe--B), in permanent magnets was disclosed in the seminal work of J. J. Croat, et al. (J. Appl. Phys., 55(6), 2078 (1984) and M. Sagawa, et al. (J. Appl. Phys., 55(6), 2083 (1984)). Refinements of this process usually recognize that for the reduction/diffusion method to be most effective it is important to prevent the rare earth metal from being oxidized as processing proceeds and to remove residual calcium as completely as possible.
One line of development is set forth in Japanese patents JP 62004807, JP 62004806, JP 61295308 and JP 61270303 which all disclose the addition of alkaline earth metal chlorides to the starting materials for the reduction/diffusion method. Alkaline earth metal chlorides have low melting points and form a liquid phase during the reduction/diffusion process, which allows the alkaline earth metal chlorides to permeate into the grains of the reduction/diffusion reaction product. As a result of this permeation, the reduction/diffusion reaction product, which contains the desired R--Fe--B alloy, will disintegrate more completely to form individual particles during the wet process, the wet process being the subsequent steps involving water and, if necessary, acid. This level of disintegration, which stems from the use of alkaline earth metal chlorides, facilitates the removal of the residual calcium. However, although the alkaline metal chlorides in the reduction/diffusion method effectuates the eventual removal of calcium contaminant, their use causes other problems.
One disadvantage in adding alkaline earth metal chlorides to the starting materials is the difficulty they cause in controlling the size of the individual alloy particles. The size of the individual alloy particles is important because it determines the extent of any subsequent oxidation the particles may undergo and further determines the extent of calcium removal. If the particle diameter is below 10 microns (.mu.m), it will be more readily oxidized, leading to degradation of magnetic properties. If the particle size is too large, it will be difficult to remove residual calcium.
Another disadvantage to the alkaline earth metal chloride technique stems from the low melting points of the chlorides. Low melting points lead to the contamination of the reduction/diffusion furnace which adversely affects product quality and is disruptive to overall processing.
Hence there is a continuing need for improvements in methods for preparing rare earth-iron-boron alloy powders.